The objective of this research is to characterize the interaction of viral transforming proteins with their cellular targets, and to determine the role of these targets in the process of transformation. A number of cellular polypeptides appear to be phosphorylated by the Rous sarcoma virus (RSV) transforming gene product, pp60(src). These polypeptides are also phosphorylated by the protein kinase activity associated with the transforming protein of Fujinami sarcoma virus (FSV), P140. The FSV-encoded P140, and the cellular substrates of pp60(src) and P140, will be purified and characterized biocehmically, and their cellular localization determined by cell fractionation and immunocytochemical techniques. To investigate the role of these cellular polypeptide substrates in transformation, revertants of cells transformed by RSV will be isolated and characterized. In addition, the mechanisms involved in dephosphorylating the phosphorylated polypeptides will be examined. These studies will provide information about the role of protein phosphorylation in malignant transformation, and will form the basis for future studies on growth control in transformed cells.